In mobile communications systems, a location update procedure is performed to allow a roaming of a subscriber from one to another service domain. E.g., if a mobile subscriber, who is registered in a service domain of his Home Public Land Mobile Network HPLMN operator, leaves his home country and moves to a service domain operated by a Visited Public Land Mobile Network VPLMN operator, the location update procedure is performed to allow for the international roaming. Details about service accessibility can be found, e.g., in 3G TS 22.011 v.3.3.0, 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Service accessibility (Release 1999).
The term ‘roaming’ describes as well a scenario, wherein the mobile subscriber has currently not accessed his HPLMN domain, e.g. because the terminal is at present switched off, but accesses while being located in a VPLMN domain the VPLMN domain by switching his terminal on.
For UMTS, it is planned to use GSM Subscriber Identity Modules SIMs in a UMTS mobile terminal to get access to service. However, the current standards for roaming in UMTS and GSM as well as the legal requirements set by individual national countries, differ with respect to roaming for the subscriber. UMTS subscribers will be entitled to national roaming, as it is foreseen that the UMTS networks initially consist of small islands only. To provide more frequent access to service, the subscriber shall have the possibility to register to networks on a national basis in addition to international roaming.
GSM subscribers are usually not entitled to national roaming. Nevertheless, the GSM subscriber using a dual mode, e.g. GSM-UMTS, mobile terminal shall be able to access both GSM and UMTS systems in his home country as well as abroad.
Currently, it is not possible to allow for a differentiated accessing of dedicated service domains according to the outlined roaming rules, as there is no means to distinguish GSM subscribers from UMTS subscribers. The reason is that for the 3GPP Release 99 and beyond, both GSM and UMTS apply the same security mechanisms as stated in 3G TS 33.102, v.3.6.0, 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; 3G Security; Security Architecture, clause 6.8.1.1.
Problems occur for roaming cases involving a Home Public Land Mobile Network HPLMN operator and a Visited Public Land Mobile Network VPLMN operator if both of them are running GSM and UMTS networks in the same country and are using the same Mobile Network Codes MNCs for their networks. Such a scenario is likely in particular in the early days of UMTS introduction when operators run their UMTS networks from an administration point of view as extensions of their existing GSM networks.
If a GSM subscriber being served by his HPLMN operator's UMTS network moves, he can get out of coverage of his HPLMN operator's UMTS network but in coverage of another UMTS operator's network. According to national roaming agreements for UMTS he is granted service by the visited UMTS network. If getting out of coverage of the visited UMTS network, he is served by the same network operator's GSM network. Although there might be no roaming agreement for GSM existing between the GSM networks, the GSM subscriber cannot be prevented from roaming to the foreign GSM network.